This invention relates to a glass bottle forming machine of a press blow type which forms a parison by pressing gob produced from molten glass and forming a glass bottle of a desired shape by subjecting this parison to a blow processing.
Known in the art of glass bottle forming machines producing glass bottles in a large scale is a glass bottle forming machine of an independent section type (IS type) having forming sections of a press blow type.
Each forming section of this IS type glass bottle forming machine includes parison forming means for forming a parison by pressing gob by a plunger mechanism and blow mold forming means for forming this parison into a glass bottle of a desired shape by the blow processing.
A glass bottle formed in each forming section including the parison forming means and blow mold forming means passes through annealing furnace one by one and thereby is finished to a final glass bottle. A check is made at the outlet of the annealing furnace to examine whether there is a defect in the finished bottle or not and a glass bottle having a defect is removed out of the line.
A defect occurring during manufacture of glass bottles is mostly ascribable to variation in the weight of a gob supplied to each forming section during forming of a parison. Therefore, by preventing variation in the weight of a gob and controlling the weight of a gob supplied to each forming section to a constant value, occurrence of defective glass bottles can be held to the minimum.
As a glass bottle forming machine capable of preventing variation in the weight of a gob, there is one disclosed in Japanese Patent Publication No. 59-43426. According to this glass bottle machine, the maximum stroke value of a plunger which applies a predetermined pressure to a gob in a blank mold to form a parison is detected by a sensor such as a differential transformer, the weight of a gob is calculated from the maximum stroke value and the height of clay tube is automatically controlled so as to maintain the weight of a gob at a constant value. As another glass bottle forming machine for controlling the weight of a gob to a constant value, there is one disclosed in Japanese Patent Publication No. 1-242425.
The value of maximum stroke of a plunger during press forming depends upon temperature of molten glass stored in a spout before forming of a gob, i.e., temperature of a gob in a blank mold, for specific gravity of a gob differs depending upon the temperature of the gob and, accordingly, difference in absolute volume arises in the same weight of gob.
Accordingly, in a case where a gob of the same weight is set in a blank mold and the maximum stroke value of the plunger during forming of a parison is detected, the maximum stroke value of a gob which is of a higher temperature is smaller than the maximum stroke value of a gob which is of a lower temperature. It is therefore difficult to maintain the weight of the gob at a constant value by detecting, as in the prior art glass forming machine, the maximum stroke value of the plunger and controlling the height of the clay tube on the basis of the detected value.
Defects occur during manufacture of glass bottles not solely due to the above described variation in the weight of a gob but also due to an error in the timing of pressing by the plunger during forming of a parison and an error between center positions of the plunger and the blank mold. Accordingly, it is difficult to effectively prevent occurrence of defects in glass bottles even if the variation in the weight of a gob is prevented and the weight of a gob supplied to each forming section is maintained at a constant value, unless the timing of pressing by the plunger and also the center positions of the plunger and the blank mold are controlled to optimum values.